Wind turbine blade provided with flow altering devices

Abstract

A blade for a rotor of a wind turbine is divided into a root region closest to the hub and an airfoil region with a lift generating profile furthest away from the hub and a transition region. A base part of the transition region has an axial induction factor, which without flow altering device deviates at least 5% from a target axial induction factor. A number of first flow altering devices are arranged so as to adjust the aerodynamic properties of a longitudinal segment of the transition region to substantially meet the target axial induction factor.

The invention claimed is: 
     
       1. A blade for a rotor of a wind turbine having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade comprising:
 a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an incident airflow, 
 the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil region, wherein 
 the airfoil region comprises at least a first longitudinal segment extending along at least 20% of a longitudinal extent of the airfoil region, the first longitudinal segment comprising a first base part having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, characterised in that 
 the first base part has an axial induction factor, which without flow altering device deviates at least 5% from a target axial induction factor at a design point, and 
 the first longitudinal segment is provided with a number of first flow altering devices arranged so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target axial induction factor at the design point. 
 
     
     
       2. A blade according to  claim 1 , wherein the flow altering devices comprises devices chosen from the group of:
 multi element sections, such as a slat, or a flap, and/or 
 surface mounted elements, such as a leading edge element or a surface mounted flap, which alters an overall envelope of the first longitudinal segment of the blade. 
 
     
     
       3. A blade according to  claim 2 , wherein the flow altering devices in addition comprises boundary layer control means, such as holes or a slot for ventilation, vortex generators and a Gurney flap. 
     
     
       4. A blade according to  claim 1 , wherein the first base part has an inherent non-ideal twist and/or chordal length, and wherein the cross-sectional profile is adapted to compensate for the non-ideal twist and/or chordal length by shifting the axial induction towards the target axial induction. 
     
     
       5. A blade according to  claim 1 , wherein the axial induction factor of the first longitudinal segment with flow altering devices deviates no more than 2%, or no more than 1% from the target axial induction factor at the design point. 
     
     
       6. A blade according to  claim 1 , wherein the induction factor of the first base part without flow altering means deviates from the target axial induction factor along substantially the entire longitudinal extent of the first longitudinal segment. 
     
     
       7. A blade according to  claim 1 , wherein the target axial induction factor is substantially equal to the aerodynamic optimum target axial induction factor. 
     
     
       8. A blade according to  claim 1 , wherein the target axial induction factor lies in the interval between 0.25 and 0.4, or between 0.28 and 0.38, or between 0.3 and 0.33. 
     
     
       9. A blade according to  claim 1 , wherein the induction factor of the first base part without flow altering devices at the design point deviates at least 10%, or 20% or 30% from the target axial induction factor. 
     
     
       10. A blade according to  claim 1 , wherein the first base part without flow altering devices at the design point further deviates from a target loading, and wherein the first flow altering devices are further arranged so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target loading at the design point. 
     
     
       11. A blade according to  claim 10 , wherein the loading of the first base part without flow altering devices at the design point deviates at least 5%, or 10%, or 20% or 30% from the target loading. 
     
     
       12. A blade according to  claim 10 , wherein the loading of the first longitudinal segment with flow altering devices deviates no more than 2%, or no more than 1% from the target axial induction factor at the design point. 
     
     
       13. A blade for a rotor of a wind turbine having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade having a predetermined target axial induction factor at a rotor design point, the blade comprising:
 a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an incident airflow,
 the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil region, characterised in that 
 
 the airfoil region is divided into a number of base sections, a first one of said base sections extending along at least 20% of a longitudinal extent of the airfoil region, the first base section having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge,
 the first base section being formed so as deviate at least 5% from the target axial induction factor at the rotor design point, and 
 the first base section being provided with a number of first flow altering devices arranged so as to adjust the aerodynamic properties and substantially meet the target axial induction factor at the rotor design point. 
 
 
     
     
       14. A method of designing a blade with a predetermined target axial induction factor at a rotor design point for a rotor of a wind turbine having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade comprising:
 a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an incident airflow, 
 the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil region, wherein 
 the airfoil region comprises at least a first longitudinal segment extending along at least 20% of a longitudinal extent of the airfoil region, the first longitudinal segment comprising a first base part having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, characterised by 
 designing the first base part with an axial induction factor deviating at least 5% from the target axial induction factor at the rotor design point, and 
 designing and arranging a number of first flow altering devices at the first longitudinal section so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target axial induction factor at the rotor design point. 
 
     
     
       15. A method of modifying a blade for a rotor of a wind turbine having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade comprising:
 a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an incident airflow, 
 the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil region, wherein 
 the airfoil region comprises at least a first longitudinal segment extending along at least 20% of a longitudinal extent of the airfoil region, the first longitudinal segment comprising a first base part having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, characterised by 
 the first base part being designed with an axial induction factor, which without flow altering device deviates at least 5% from a target axial induction factor at a design point, and 
 modifying the first longitudinal segment by providing a number of first flow altering devices arranged so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target axial induction factor at the design point.